1. Field of the Invention
The present invention relates to a carrier for developing electrostatic latent images formed on an image-bearing member such as a photosensitive member in image forming apparatus such as copying machines, printers and the like, and specifically relates to a binder-type carrier having magnetic powder dispersed in resin for developing electrostatic latent images.
2. Description of the Related Art
Conventionally, in image forming apparatuses such as copying machines, printers and the like, an electrostatic latent image formed on an image-bearing member such as a photosensitive member is commonly developed using a mixture of carrier and toner as a developer.
Examples of well-known carriers mixed with toner include conventional powders such as iron, ferrite and the like used directly, as well as binder-type carriers having such magnetic powder dispersed in resin.
Carriers that use iron, ferrite or the like directly generally have low electrical resistance which results in disadvantages when such carriers are used for developing inasmuch as the electrical load on the surface of the image-bearing member flows through the carrier and produces white spots in the developed image, and the carrier adheres to the image-bearing member due to the electrical load injected from the developing sleeve. Furthermore, the head of the magnetic brush formed by such carriers are generally hard, leading to disadvantages such as streaks when developing halftone images such as photographic documents and the like.
Thus, in recent years attention has focused on the aforesaid binder-type carriers wherein magnetic powder is dispersed in resin.
Such binder-type carriers having magnetic powder dispersed in resin typically have a weak magnetic force, however, which weakens the magnetic restraint exerted on said carrier by a magnetic roller or the like, such that the carrier is released from the developing sleeve and adheres to the image-bearing member. Disadvantages arise from this situation such as generation of noise in formed images, and damage to the image-bearing member caused by adhered carrier and the like.
When large a large amount of magnetic powder dispersed in resin is included in carriers of the aforesaid binder type, therefore, much of said magnetic powder may be exposed on the surface of the carrier and reduce the resistance value of the carrier. During development, the electrical charge on the image-bearing member may flow through the carrier and produce undeveloped white spots in the formed image. Furthermore, bonding between the resin and magnetic powder may be adversely affected by the large content of magnetic powder, thereby causing the carrier to readily breakdown.
Heretofore, carriers have been developed which provide a surface coat of resin over the entire surface of a binder-type carrier having a large amount of magnetic resin powder dispersed in resin, such as disclosed in Japanese Unexamined Patent Application No. SHO 58-59457. The provision of a resin surface coating suppresses the breakdown of the binder-type carrier, and the addition of electrically conductive or charge-control agents to said surface coating allows the resistance value of the entire carrier to be regulated, as well as to regulate chargeability relative to the toner.
When a resin surface coating is provided on the entire surface of a binder-type carrier, however, the surface of the carrier is formed by a composite surface of resin and magnetic powder which has numerous charge points relative to the toner, thus losing an advantage of the binder-type carrier which has durability with respect to spent carrier, and becoming unable to suitably charge the toner. The addition of agents having conductive properties and charge-controlling properties to the resin surface coating is disadvantageous inasmuch as it is troublesome and complicated, and increases production costs.